DE 35 42 681 A1 describes a method for producing high-quality drawn tubes, in which a metal strip is shaped into a tube with a longitudinal slit, and the longitudinal slit is welded. In a final operation, the welded metal tube is reduced in its outside diameter and its wall thickness with a drawing ring and a floating mandrel in a continuously operating tube-drawing machine.
When the tube-drawing machine is being set up, especially with respect to the selection of the drawing tools (mandrel, die), the wall thickness of the reduced metal tube must be measured before production can begin. In many cases, this can significantly delay the beginning of production.
In the continuous production of metal tubes with an outside diameter of less than 5 mm and a wall thickness of less than 0.1 mm, the wall thickness has been measured by optical means up until now. In this measuring operation, a short piece of the finished tube is embedded in casting resin, and after the casting resin has cured, a microsection of the surface is prepared, which can then be viewed under a microscope. A photograph of the transverse microsection is taken, and the tube is measured.
A high-precision microscope with a camera adapter and suitable measuring software are necessary for the extremely thin metal tubes under discussion here. This apparatus is very expensive, and the operation is very time-consuming, i.e., it can take several hours. During this period of time, production cannot be started.